A European eel (Anguilla anguilla) case study using structured elicitation to estimate instream infrastructure passability for freshwater fishes

Abstract

Abstract Conservation efforts are hampered by limited understanding about how different types of instream infrastructure impact migration patterns and fish survival. We used a rapid, fully online IDEA protocol to elicit expert judgments for the passability of seven different in‐stream infrastructures to elver European eels (Anguilla anguilla) in Great Britain. Nine experts provided judgments via our online survey, followed by a second elicitation via email for reflection and adjustment of initial estimates. We found that on average, bridges were judged the most passable (95% passability), followed by fords, nonperched culverts, weirs, sluices, dams, and perched culverts (7%). Results showed a high degree of agreement about how passable bridges and perched culverts are for elver eels, but less certainty about other infrastructure. Thirty‐four distinct factors were identified that experts believed influence infrastructure passability for elver eels, including: the structure itself, hydraulics, elver characteristics, obstructions (e.g., debris accumulation), and vegetation (e.g., to aid climbing). We discuss how our rapid, online‐only variation on the IDEA protocol compares with the more traditional protocol, and how the expert estimates generated in this study can be used in future scenario building and connectivity modeling, with a view to improving conservation to support species persistence.